1. Field of the Invention
The present invention relates to substantially pore-free boron carbide sintered bodies with a density of not more than 2.60 g/cm3 and improved mechanical properties, and to a process for their manufacture.
2. Background Art
Boron carbide, B4C, is a lightweight solid (density, 2.52 g/cm3) that has high hardness and a high resistance to abrasive wear and has been used mainly as an abrasive. In dense, sintered form it has been applied as armor for bulletproof body vests, for vehicles and aircraft, as wear resistant linings such as sand blasting nozzles and as control rods in nuclear reactors.
Densification of boron carbide to relative densities of above 95% TD (theoretical density) typically requires small additions of amorphous carbon as a sintering aid and takes place at temperatures of at least 2100° C. Nevertheless for full densification (>99% TD) a hot pressing treatment is required. Boron carbide has the disadvantage of high brittleness, i.e. monolithic boron carbide ceramics have a very low fracture toughness which varies between 2.1 and 2.6 MPa·m1/2.
Self-bonded boron carbide has therefore not hitherto become established in applications as a structural ceramic, where strong and tough components are required. Fracture toughness of boron carbide armor should be improved, since according to LaSalvia [Ceram. Sci. and Eng. Proceedings 23, 213-220 (2002)] the armor ceramics should have both high hardness and high toughness to prevent penetration of the projectile.
Attempts have therefore been made to reinforce boron carbide, like other brittle monolithic ceramics, by dispersion of particulate hard materials. Thus, U.S. Pat. No. 5,543,370 to Sigl et al., for example discloses the toughening of boron carbide by the addition of titanium diboride (TiB2) and free carbon. The solid state sintered and HIP post-densified composites (HIP-conditions 2100° C., 200 MPa argon pressure) with 20 and 40 vol-% TiB2 exceed both the toughness and also the strength of pure boron carbide, with four-point flexural strength values in the range of from 550 to 740 MPa and KIC values in the range of from 4.7 to 6.8 MPa·m1/2. The sintered densities of these B4C based composites varied significantly with the amount of TiB2, i.e. 2.90 g/cm3 for 20 vol-% TiB2 and 3.30 g/cm3 for 40 vol-% TiB2. In a similar way to titanium diboride, other borides of the transition metals of the groups 4a to 6a of the Periodic Table, in equilibrium with B4C can also be used to improve the mechanical properties of boron carbide. EP 1,452,509 Al to Hirao et al., discloses a boron carbide chromium diboride (CrB2) composite, sintered at 2030° C. containing a dispersion of 10 to 25 mol-% CrB2 particles, and having a 4-point flexural strength in the range of 436 to 528 MPa and a fracture toughness of at least 3.0 MPa·m1/2, respectively. While the above mentioned composite materials are formed by mixing the desired metal boride phases and subsequent sintering, processes have also been described in which the desired metal boride composition is only formed after a suitable reaction of the starting materials during the sintering step. Skorokhod and Krstic [“High Strength—High Toughness B4C—TiB2 Composites”, J. Mater. Sci. Lett., 19, 237-239 (2000)] have successfully fabricated a 85 B4C-15 TiB2 (vol-%) composite with a flexural strength of 621 MPa and a fracture toughness of 6.1 MPa·m1/2 (measured by the SENB method with a 100 μm notch width) by reaction hot pressing of a sub-micron particle sized boron carbide powder using additions of sub-micron size TiO2 and carbon at a temperature of 2000° C. The formation of uniform distributed TiB2 particles (<5 μm grain size) was in accordance with the reaction(1+x)B4C+2 TiO2+3 C→xB4C+2 TiB2+4 CO
The high strength of this material was attributed to the combination of high fracture toughness and fine microstructure. A further improvement of the mechanical properties of 80 B4C—20 TiB2 (mol-%) composites is disclosed in EP 1,452,509 A1 to Hirao et al., wherein via use of nanometer size TiO2 powder, carbon black and sub-micron particle sized B4C powder after reaction hot pressing at 2000° C. with a very high pressure of 50 MPa, dense sintered bodies (density 2.82 g/cm3, 100% TD) with both high flexural strength (720-870 MPa) and high toughness (2.8 to 3.4 MPa·m1/2, SEPB method) could be obtained. The improvement of mechanical properties was attributed to the fine-grained microstructure and uniform dispersion of TiB2 particles.
However, the proposed toughening method by dispersion of metal borides has technological disadvantages in view of the used densification processes (1) and concerning other material properties of the densified end product (2).
(1) Densification Processes
Solid state sintering of B4C—TiB2 composites requires high sintering temperatures in the range of from 2000 to 2175° C. Sintering of B4C—CrB2 composites is possible at 2030° C., however densification is incomplete (residual porosity above 2%). Via reaction hot pressing at 2000° C. 100% dense B4C—TiB2 composites can be obtained, however, the high molding pressure of 50 MPa used (see [0056] in EP 1,452,508 to Hirao et al.) restricts hot pressing to small area parts. Moreover, homogeneous distribution of ultra-fine TiO2 and carbon black in a methanol-B4C dispersion and drying of the flammable slurry are delicate processes and difficult to scale-up.
(2) Other Material Properties of Densified End Product
Since for toughening by particle dispersion the optimum volume content of added or in-situ grown particles is relatively high, densities of composites were increased, e.g., to 2.82 g/cm3 for a 15 vol-% TiB2 and to 3.32 g/cm3 for a 40 vol-% TiB2 composite, respectively. However, for lightweight armor application the density should remain as low as possible (below 2.60 g/cm3). Further, since hardness of TiB2 is significantly lower (comparable only to SiC), the resulting hardness of the B4C—TiB2 composites is inferior to the commercial grades of monolithic B4C ceramics. Therefore, the relatively high densities combined with a lower hardness inhibits the use of tough B4C—TiB2 composites as a lightweight ceramic armor material.
Another approach to produce tough and high strength B4C ceramics is to use liquid phase sintering. Lee and Kim [J. Mat. Sci. 27 (1992), 6335-6340] have shown that the addition of alumina, Al2O3, promoted the densification of boron carbide and a maximum density of 96% of theoretical can be obtained with 3 wt-% alumina-doped B4C sintered at 2150° C., i.e. above the melting point of Al2O3. The microstructure showed equiaxed B4C grains with a mean grain size of about 7 μm. However, as the addition of Al2O3 exceeded 3 wt-% exaggerated grain growth occurred, which was attributed to the liquid phase.
It has been reported by Kim et al., [J. Am. Ceram. Soc. 83, No. 11, 2863-65 (2000)], that by hot pressing of B4C with alumina additions up to 5 vol-% at 2000° C. the mechanical properties can be remarkably increased as compared to undoped, hot pressed B4C of 88% relative density. Fracture toughness increased steadily with the addition of Al2O3 from ˜3 MPa·m1/2 (2.5 vol % Al2O3) up to 3.8 MPa·m1/2 (10 vol-% Al2O3). However the achieved flexural strength was below 560 MPa.
The use of yttria (Y2O3) containing sintering aids was first described in two Japanese Patent Applications, JP 62012663 to Kani (pressureless sintering of B4C with mixed additions of 4 wt-% Al+1 wt-% Si+3 wt-% Y2O3 at 2000° C.) and JP 08012434 to Kani (pressureless sintering with 0.5 wt-% Al+3 wt-% Y2O3 at 2000° C.). It was shown that instead of Y2O3 one can also use other oxides, nitrides, carbides or borides, the net result being the same. However, these processes are complicated due to sintering in atmospheres containing high aluminum partial pressures. Furthermore, no improvement in fracture toughness of B4C materials was reported.
The possibility to improve fracture toughness of boron carbide with yttria or mixed additions of yttria in combination with other oxides was first demonstrated in U.S. Pat. No. 5,330,942 to Holcombe et al., and CN 1,438,201 to Li et al., respectively.
According to the method with is disclosed in U.S. Pat. No. 5,330,942, the fracture toughness of B4C can be increased to 3.9 MPa·m1/2 by vacuum sintering at 1900 to 1975° C. using powder compacts of composition 97.5 B4C-2.5 carbon (wt-%) packed in a yttria grit of 0.15 to 1.4 mm grain size. The vacuum allows yttrium oxide vapor to penetrate the powder compact promoting reaction-sintering of carbon-doped B4C to full density (2.62 g/cm3). The final composite showed an overall yttrium content of 9.4 wt-%, the yttrium being present in the form of Y—B—O—C containing 5 μm particulates dispersed evenly in a matrix of 40 μm boron carbide grains. X-ray diffraction identified that yttrium boride and yttrium borocarbide coexist with B4C. However, owing to uncontrolled gas infiltration this method is as yet unsuitable for mass production of liquid phase sintered B4C.
The Chinese patent application CN 1,438,201 also discloses a method to increase the toughness of boron carbide while maintaining a reasonable hardness and intermediate strength. The basis of the method is to use liquid phase sintering under vacuum or streaming argon of powder compacts comprising B4C powder (average particle size 0.6 to 3.5 μm) and 1 to 28 wt-% additions of Y2O3 in combination with Al2O3 or aluminum nitride (AlN) and any one of La2O3 or CeO2 components. The B4C material of example 4 (starting from a sub-micron powder mixture 95.2 B4C-0.8 La2O3-1.7 AlN-2.3 Y2O3, wt-%) obtained by pressureless sintering at 1920° C. for 270 minutes (i.e. 4.5 hours hold at max. temperature) was characterized with regard to mechanical properties: Vickers hardness 2950, four-point bend strength 520 MPa and fracture toughness 5.4 MPa·m1/2. No indication of the microstructure and the chemical composition of the final B4C sintered bodies was given. However, with the proposed method of pressureless sintering, in particular in view of the used atmosphere (vacuum/streaming argon gas), the long hold times at temperatures of around 1900° C., it is necessary to consider reactions between boron carbide and the rare earth oxides of the liquid phase. Reactions accordingY2O3+3 B4C→2 YB6+3 CO andLa2O3+3 B4C→2 LaB6+3 COresult in formation of rare earth borides, in weight losses (evolution of carbon monoxide) and in a decrease in sintered density of the bodies. Due to this, production of large parts using this process would be difficult to control. This is supported by experiments of the present inventors who, while reproducing the example 4 of CN 1,438,201, obtained very porous bodies (sintered density of only 2.05 g/cm3, corresponding to 80% of theoretical density). The obtained samples were found to contain rare earth hexaboride (LaB6) and rare earth borocarbide (YB2C2) in addition to boron carbide. Thus, due to the decomposition reactions the liquid phase was depleted to such an extent that is was not possible to make dense bodies.
The present invention differs from the teachings of U.S. Pat. No. 5,330,942 and CN 1,438,201 relating to (1) a low-temperature low-pressure hot-pressing densification without any appreciable reaction between B4C and the liquid phase, (2) a new B4C material containing a rare earth aluminate as main component of the oxide binder phase, and having unique mechanical properties.